The treatment of cancer with radiation therapy is profoundly impacted by the certainty with which the exact locations of the target and surrounding structures are known. The spatial precision with which radiation therapy can be delivered has surpassed the precision with which the location of many anatomical structures can be determined. Organ motion due to respiration requires that the time dependence of internal anatomy be accurately and precisely described. Precise measurement of the temporal characteristics of the human respiratory cycle, and the corresponding time variation in internal organ motion, will lay a foundation upon which dynamic imaging, treatment planning and treatment delivery in radiation therapy can be designed. This project will establish a broadly applicable criterion for the minimum rate at which dynamic images need to be acquired in order to fully describe the temporal characteristics of respiratory-induced organ motion. This will be accomplished via the synchronization of spirometry and dynamic ultrasound images. The minimum image sampling rate, and the phase relationship between flow volume and organ displacement, will be measured. These results will be applicable to any dynamic imaging modality. Ultrasound is unique among imaging modalities both in its readiness for dynamic imaging and in its difficulty in interpretation. This project will develop a system that superimposes a dynamic US image onto a conventional CT image, both in real time and in a "simulation" mode. The image superposition will be accomplished through the use of a unique multi-layer display technology. The spatial position and orientation of the ultrasound image will be accurately registered via an articulating arm. A unique user interface will be developed to facilitate image interpretation. The dual-modality imaging system will be clinically tested on patients receiving radiation therapy for cancers in the pelvis, abdomen, and thorax. This imaging tool will enable radiation oncology physicians to precisely localize tumor and critical structure boundaries, both in space and in time. The interpretation of the dynamic ultrasound images will be feasible to the novice via the context provided by the superposition of the US and CT images. The proposed system will find application in the areas of image-guided biopsy, brachytherapy, and education, as well as in radiation therapy treatment planning and daily verification.